1. Field of the Invention
This invention relates to crosslinked polyvinylpyrrolidone (PVP), and more particularly, to strongly swellable, moderately crosslinked PVP polymers which are prepared directly as fine, white powders by precipitation polymerization of vinylpyrrolidone (VP).
2. Description of the Prior Art
Polyvinylpyrrolidone is made by homopolymerization of vinylpyrrolidone in bulk, solution or suspension. The polymer is obtained as (1) water soluble PVP; (2) water insoluble PVP which is variably swellable in water and which may form soft gels if strongly swellable.
(1) Water soluble PVPs usually are polymerized in aqueous solution using hydrogen peroxide. Polymerization also can take place in organic solvents such as methanol, isopropanol, toluene, benzene, tetrahydrofuran, acetone and hexane, usually with the production of lower molecular weight polymers than polymerization in water.
Belgian Patent 668,368, for example, disclosed a process for preparing homo- and copolymers of vinyl lactams in aliphatic hydrocarbons. However, the patent was limited to water soluble polymers and copolymers of vinyl pyrrolidone and vinyl caprolactam without crosslinking agents.
(2) Water insoluble polymers of VP which are variably swellable may be prepared by copolymerizing VP in the conventional manner with or without difunctional vinyl or acrylic compounds acting as crosslinking agents. However, even when different amounts of the difunctional component was used, it was not possible to prepare polymers which were only slightly swellable in water.
U.S. Pat. No. 2,938,017, for example, disclosed that insoluble polymerization products can be obtained by heating VP alone, without the addition of water but in the presence of basic alkali or alkaline earth metal compounds such as the oxides, hydroxides or alkoxides of sodium or potassium. This method produced slightly swellable polymers, which, however, showed strong discoloration due to the high temperature at which polymerization took place. Moreover, the reaction was very difficult to control under these conditions.
U.S. Pat. Nos. 3,277,066 and 3,306,886 disclosed processes for the manufacture of insoluble PVP in which VP was placed in an autoclave and heated under pressure at about 140.degree. C. in the presence of water and catalysts such as alkali metal hydrides, alkali metal borohydrides and/or alkali or alkaline earth metal hydroxides or alkoxides until the polymerization reaction had started. These processes also produced only sparingly swellable polymers with the disadvantage that elevated temperatures were required to start the polymerization reaction, and this necessitated the use of pressure vessels when water was included.
German Patent No. 2,437,640 disclosed that VP can be polymerized in dilute aqueous solution in the presence of a crosslinking agent in the absence of oxygen and additionally in the presence of from 0.05 to 2% by weight, based on VP, of one or more ketocarboxylic acids or their methyl or ethyl esters. A small amount of a base was used concommitantly. However, polymerization at the high pH of the reaction caused yellowing of the polymers.
U.S. Pat. No. 3,933,766 described the preparation of insoluble PVP which was only slightly swellable by the so-called "popcorn" or proliferous polymerization technique. In this method, VP was polymerized to insoluble, slightly swellable, crosslinked PVP by heating an aqueous solution of VP in the presence of a bifunctional cyclic acid amide, but without any radical initiator, at about 80.degree.-100.degree. C.
U.S. Pat. No. 4,451,582 disclosed a process for the preparation of insoluble, particulate PVP polymers which were only slightly swellable in water. These polymers were obtained from basic vinyl heterocycles having a pKa higher than 4, and their copolymers, with as much as 30% by weight of copolymerizable monomers. The monomers were polymerized in the presence of from 0.1 to 10% by weight, based on the total amount of monomers, of a crosslinking agent, in the absence of oxygen and polymerization initiators. The polymers were preferably prepared by precipitation polymerization in water; however, they could be obtained in the absence of solvents for the monomers, by heating the latter to, preferably, 150.degree.-180.degree. C. This reaction was difficult to control and gave only low yields and relatively highly contaminated products.
U.S. Pat. No. 3,759,880 described a process in which VP was polymerized in aqueous solution in admixture with monomeric cyclic acid amides containing at least two ethylenically unsaturated groups of which at least one was attached to an amide nitrogen atom. The reaction was initiated in the presence of metals susceptible to attack by oxygen or in the presence of polymer seeds obtained from the monomers in the presence of such metals.
U.S. Pat. No. 4,647,637 disclosed a process for making insoluble PVP powders which were only slightly swellable by polymerization of VP and a crosslinking agent in a powder bed in the presence of a heat-transfer medium which was inert to the reactants, and in the absence of oxygen or polymerization initiator, at from 90.degree. to 220.degree. C., while maintaining the reactants in the powder state, circulating the reaction mass and evaporating the heat-transfer medium from the polymerization zone.
U.S. Pat. No. 2,658,058 disclosed a process for crosslinked linear PVP polymers by treatment of the polymer with hydrazine and hydrogen peroxide; U.S. Pat. No. 3,350,366 used .alpha.,.omega.-diolefins and hydrogen peroxide; U.S. Pat. No. 2,658,058 used persulfate; and in J. Phys. Chem. 63, 1852 (1959), crosslinking was effected by irradiation.
U.S. Pat. Nos. 4,330,451 and 4,333,112 disclosed a post-treatment process for rendering PVP more highly viscous. In this process, PVP was subjected to a heat treatment in the presence of a water-insoluble peroxide and in the absence of oxygen. However, this process suffered from the disadvantage of requiring an additional step to produce the desired high-viscosity PVP, and this step was difficult to carry out reproducibly.
Polymer J. 17 (1) p. 143-152 (1985), entitled "Polymers of N-Vinylpyrrolidone:Synthesis, Characterization and Uses", disclosed that slightly swellable, crosslinked PVP can be made by polymerizing VP chemically in aqueous solution with free radical initiators using various amounts of bifunctional monomers as crosslinking agents.
U.S. Pat. No. 3,669,103 described a process for preparing gels of lightly crosslinked PVP by aqueous, solution polymerization of VP in the presence of crosslinking agents. The reaction product was a coherent mass of gel which was evaporated to dryness and ground to provide a finely particulate product.
However, these and other processes did not provide strongly swellable, moderately crosslinked PVP polymers directly as fine, white powders which could be used to thicken aqueous and non-aqueous solutions effectively.
Accordingly, it is an object of this invention to provide strongly swellable, moderately crosslinked PVP polymers directly as fine powders by precipitation polymerization of VP in the presence of a predetermined amount of a multifunctional crosslinking agent and a free radical initiator in an organic solvent.
Yet another object of this invention is to provide strongly swellable, moderately crosslinked PVP polymer powders characterized by an aqueous gel volume of about 15-150 ml/g of polymer and a Brookfield viscosity in 5% aqueous solution of at least 10,000 cps.
Still another object of the present invention is to provide crosslinked PVP polymer powders which can show favorable white, fine powder formation during polymerization, and which can be made in a high solids content, and which provide clear solutions of high viscosities, and enhanced stability as measured by their retention of viscosity during storage at 50.degree. C., and excellent salt tolerance.
These and other objects and features of the invention will be made apparent from the following description of the invention.